This invention relates to lasers, and in particular to a method and apparatus of simultaneously generating up to twenty or more tunable WDM channels using a single-striped Ga As/AlGaAs type semiconductor optical amplifiers. This invention was funded in part by the National Science Foundation ECS contract number 9522267.
Multiwavelength optical signal sources capable of generating ultrashort and highly synchronized picosecond pulses are of great interest in novel photonic networks utilizing combined optical wavelength division multiplexed (WDM) and time division multiplexed (TDM) data formats. To date, multiwavelength generation has been demonstrated by either using spectral filtering of femtosecond optical pulses, or semiconductor laser based devices. See T Morioka, K Mori, S. Kawanishi, and M. Saruwatari, xe2x80x9cMulti WDM-Channel pulse generation from a single laser source utilizing LD-pumped supercontinuum in optical fibersxe2x80x9d, PhotonTech. Lett., vol.6, no.3, 365xcx9c368, 1994. and M. C. Nuss, W. H Knox, and D. A 13. Miller, Dense WDM with femtosecond laser pulsexe2x80x9d, IEEE/LOS 1994 Annual Meeting, Boston, Mass., 1994. and L. Boivin, M. C. Nuss J. B. Stark, W. H. Knox, and S. T. Cundiff xe2x80x9c206-channel wavelength division multiplexed transmitter using a single femtosecond laserxe2x80x9d, OSAJ[EEE-LEOS Spring Topical Meeting (Ultrafast Electronics and Optoelectronics), UMB4, Lake Taho, Nev., 1997. and D. Burns, G. Hay, and W. Sibbett, xe2x80x9cDual-wavelength external-cavity semiconductor lasersxe2x80x9d CL Q""93 Digest, vol. 11, JTHA3, 444-446, 1993; and Zhu, K. O. Nyairo and I H White, xe2x80x9cDual-wavelength picosecond optical pulse generation using an actively mode-locked multichannel grating cavity laser,xe2x80x9d Photon. Tech. Lett., Vol.6, No.3, 348-351, 1994. and C. L Wang and C-L Pan, xe2x80x9cDual-wavelength actively modelocked laser-diode array with all external grating-loaded cavityxe2x80x9d, Optics Lett., vol.19, 1456-1458, 1994.
In these approaches, there are great efforts to try to generate femtosecond optical pulses and a low channel pulse rate. However, these approaches generally suffer from limited wavelength channels, and strong gain competition between the oscillating wavelengths, which is typically characteristic of semiconductor diode lasers.
Furthermore, there have been various patents of general interest in this area that also fail to overcome the problems described above. U.S. Pat. No. 4,435,809 to Tsang et al. describes a passively mode locked laser having a saturable absorber that only has a single wavelength operation mode, with multiple longitudinal modes. U.S. Pat. No. 4,446,557 to Figueroa describes a mode-locked semiconductor laser with tunable external cavity where a user adjusts the cavity length which modifies the longitudinal mode spacing to generate a single wavelength output. U.S. Pat. No. 5,115,444 to Kirkby et al. describes a multichannel cavity laser where each wavelength is generated from a common cavity with each wavelength experiencing a different optical path length. Simultaneous generation of each wavelength is not feasible since the gain competition in the final optical amplifier stage will complicate and prevent simultaneous multiwavelength generation. U.S. Pat. No. 5,228,050 to LaCourse et al. describes an integrated multiple-wavelength laser array, each wavelength having its own cavity that is length adjustable to allow lasing at different wavelengths, and requires an array of lasers for the multiple wavelength generation. U.S. Pat. No. 5,319,655 to Thornton describes a multiwavelength laterally-injecting type lasers which requires the sources to be precisely aligned to one another. U.S. Pat. No. 5,524,012 to Wang et al. describes a tunable, multiple frequency laser diode that uses a multistripe semiconductor laser array to generate several wavelengths, and requires a grazing incidence angle on the diffraction grating. Using the grazing incidence angle prevents simultaneous wavelength generation. U.S. Pat. No. 5,524,118 to Kim et al. describes a wavelength-varying multi-wavelength optical filter laser using a single pump light source, which requires using an erbium doped fiber amplifier. U.S. Pat. No. 5,561,676 to Goldberg describes a compound-cavity high power, tunable modelocked semiconductor laser, that generates a single wavelength output that does not allow for multiple wavelength generation.
Another problem with multichannel generation from femtosecond lasers is that the multiple channels are generated by spectrally filtering the laser output after the optical pulse is generated. This is inefficient because the filtering process eliminates, or throws away, energy that was used in making the optical pulse.
The first objective of the present invention is to provide an actively modelocked laser with multiple wavelength generation.
The second object of this invention is to provide for multiwavelength generation from a semiconductor diode laser source.
The third object of this invention is to provide for generating multiwavelengths simultaneously from a single stripe semiconductor diode laser source.
The fourth objective is to generate a multiplicity of wavelength channels that can be, but are not necessarily, coherent with each other.
The fifth objective of this invention is to tailor the output spectrum of a pulsed laser by performing filtering inside the cavity (not outside) to increase the overall efficiency of the multiwavelength laser.
The sixth objective is to develop a pulsed multiwavelength laser source that will be appropriate for advanced telecommunications and optical signal processing applications, such as ultrahigh speed optical sampling.
Four embodiments describe tunable multiwavelength modelocked semiconductor lasers based on a single stripe semiconductor optical amplifier (SOA). In a first embodiment, actively modelocking the single-grating-loaded external cavity semiconductor laser system, has approximately four tunable wavelength channels that can be simultaneously generated with each wavelength transmitting approximately 12 ps pulses at approximately 2.5 GHz.
In a second embodiment, actively modelocking a single-grating-loaded external cavity semiconductor laser system has approximately 20 channels that can be simultaneously generated with each wavelength transmitting approximately 12 ps pulses at a rate of approximately 600 MHz. A conventional pulse interleaving configuration multiplexes the output optical pulse train by eight fold to give a final 5 Gbit/s pulse rate. To the inventors knowledge, this is the first demonstration of a single semiconductor laser directly generating approximately 20 wavelengths simultaneously without using a super continuum. The resulting geometry provides a very compact configuration to reach an aggregate data throughput of approximately 100 Gbit/s.
In a third embodiment, a multiple wavelength generation from a single laser source is achieved using a fiber-array and grating. Finally in a fourth embodiment, multiple wavelength generation is achieved from a single laser source using a Fabry-Perot Spectral filter.